1. Field of the Invention
The present invention relates to a communication system having a plurality of nodes connected to a common transmission line.
2. Description of the Related Art
In a conventional communication system, as shown in FIG. 1, a plurality of nodes, i.e., transmission/reception circuits 31 through 3n are connected to two-wire type transmission lines 1 and 2. All of the transmission/reception circuits 31 through 3n have the same configuration. A positive potential Vcc (e.g., 5 V) is supplied to one end of the transmission line 1 through a terminating resistor 4, and the positive potential Vcc is similarly supplied to the other end of the transmission line 1 through a terminating resistor 5. A ground potential Vg (e.g., 0 V) is supplied to one end of the transmission line 2 through a terminating resistor 6, and the ground potential Vg is similarly supplied to the other end of the transmission line 2 through a terminating resistor 7.
In the transmission/reception circuit 31, bidirectional input/output filter 11 is connected to the transmission lines 1 and 2 through a connector 12. Transmission signals are separately supplied through a non-inverting amplifier circuit 13 and an inverting amplifier circuit 14 to connection terminals B1 and B2 of the input/output filter 11 located opposite to terminals A1 and A2 thereof connected to the transmission lines 1 and 2. Bias circuits 17 and 18 are connected to the connection terminals B1 and B2 of the filter 11 through respective AC coupling circuits 15 and 16 which are formed by a resistor and a capacitor. Signals provided by the bias circuits 17 and 18 are passed through a differential amplifier 19 to provide a reception signal.
When a transmission signal is output, the transmission signal is amplified by the non-inverting amplifier circuit 13 and is inverted and amplified by the inverting amplifier circuit 14. Transmission signals in opposite phases are supplied from the non-inverting amplifier circuit 13 and inverting amplifier circuit 14 to the filter 11. The filter 11 performs low-pass filtering of those transmission signals separately. The transmission signal output by the non-inverting amplifier circuit 13 is supplied to the transmission line 2 through the filter 11, and the transmission signal output by the inverting amplifier circuit 14 is supplied to the transmission line 1 through the filter 11.
A signal transmitted over each of the respective transmission lines 1 and 2 is supplied to the filter 11, and the filter 11 performs low-pass filtering of those transmission signals separately to output the signals to the AC coupling circuits 15 and 16. The AC coupling circuits 15 and 16 extract AC components in the transmission signals and supplies the components to the bias circuits 17 and 18.
For example, when a signal A transmitted over the transmission line 1 and a signal B transmitted over the transmission line 2 have respective opposite phases to each other as shown in FIG. 2A, the bias circuit 17 applies a bias voltage to the transmission signal A to provide a biased signal BIASA and the bias circuit 18 applies a bias voltage to the transmission signal B to provide a biased signal BIASB as shown in FIG. 2B. The signals output by the bias circuits 17 and 18 are detected by the differential amplifier 19 as a reception signal as shown in FIG. 2C.
When the transmission line 1 is broken, since only the signal B is transmitted over the transmission line 2, the biased signal BIASB provided by applying a bias voltage to the signal B transmitted over the transmission line 2 changes similarly to the way the signal B changes, while the biased signal BIASA remains constant without changing as shown in FIG. 2D. At the differential amplifier 19, the constant biased signal BIASA is compared with the biased signal BIASB to provide a reception signal as shown in FIG. 2E. Similar situations occur when the transmission line 1 is grounded and when the transmission line 2 is broken or grounded.
If the bias circuits 17 and 18 are not provided, the signals A and B input to the differential amplifier 19 will appear as shown in FIG. 2F when the transmission line 1 is broken, which disables the detection of a reception signal.
The configuration and operation of the transmission/reception circuits 32 through 3n, are the same as those of the transmission/reception circuit 31 described above. A conventional communication system as described above is disclosed, for example, in Japanese unexamined patent publication No. H3-171849.
As shown in FIG. 3, an equivalent circuit of the two-wire type transmission lines 1 and 2 can be represented by a resistance R0, inductance L0 and capacitance C0 per unit length.
A capacitance C1 also exists in each of the transmission/reception circuits 31 through 3n as shown in FIG. 4. The rising edge and the falling edge of a transmission signal undergo transient changes under influence of those capacitances. Referring to the transmission line 1, the resistance R of the terminating resistors 4 and 5 and the capacitance C consisting of the capacitances C0 and C1 cooperate to form a time constant to cause transient changes in the rise and fall of the waveform of a signal transmitted over the transmission line 1 as shown in FIG. 6.
However, when there is a break somewhere in the transmission line 1 for example, since the resistance R is doubled, the time constant is also doubled if any change of the capacitance C is ignored. As a result, the waveform of the signal transmitted over the transmission line 1 becomes significantly less sharp. This similarly occurs in the case of poor contact between each transmission/reception circuit and the transmission line 1, and the waveform of a transmission signal is significantly affected by the time constant in the case of line breakage or poor contact not only in the transmission line 1 but also in the transmission line 2. A problem has arisen in a transmission/reception circuit in that it is impossible to accurately obtain a reception signal from a transmission signal having a transient waveform under such influence of a large time constant without decreasing the communication speed.